compiler/rustc_trait_selection/src/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::opaque_types::required_region_bounds;
   3 use crate::traits;
   4 use rustc_hir as hir;
   5 use rustc_hir::def_id::DefId;
   6 use rustc_hir::lang_items::LangItem;
   7 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
   8 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
   9 use rustc_span::Span;
  10
  11 use std::iter;
  12 use std::rc::Rc;
  13 /// Returns the set of obligations needed to make `arg` well-formed.
  14 /// If `arg` contains unresolved inference variables, this may include
  15 /// further WF obligations. However, if `arg` IS an unresolved
  16 /// inference variable, returns `None`, because we are not able to
  17 /// make any progress at all. This is to prevent "livelock" where we
  18 /// say "$0 is WF if $0 is WF".
  19 pub fn obligations<'a, 'tcx>(
  20     infcx: &InferCtxt<'a, 'tcx>,
  21     param_env: ty::ParamEnv<'tcx>,
  22     body_id: hir::HirId,
  23     recursion_depth: usize,
  24     arg: GenericArg<'tcx>,
  25     span: Span,
  26 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  27     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  28     let arg = match arg.unpack() {
  29         GenericArgKind::Type(ty) => {
  30             match ty.kind() {
  31                 ty::Infer(ty::TyVar(_)) => {
  32                     let resolved_ty = infcx.shallow_resolve(ty);
  33                     if resolved_ty == ty {
  34                         // No progress, bail out to prevent "livelock".
  35                         return None;
  36                     }
  37
  38                     resolved_ty
  39                 }
  40                 _ => ty,
  41             }
  42             .into()
  43         }
  44         GenericArgKind::Const(ct) => {
  45             match ct.val {
  46                 ty::ConstKind::Infer(infer) => {
  47                     let resolved = infcx.shallow_resolve(infer);
  48                     if resolved == infer {
  49                         // No progress.
  50                         return None;
  51                     }
  52
  53                     infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Infer(resolved), ty: ct.ty })
  54                 }
  55                 _ => ct,
  56             }
  57             .into()
  58         }
  59         // There is nothing we have to do for lifetimes.
  60         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  61     };
  62
  63     let mut wf =
  64         WfPredicates { infcx, param_env, body_id, span, out: vec![], recursion_depth, item: None };
  65     wf.compute(arg);
  66     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  67
  68     let result = wf.normalize();
  69     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  70     Some(result)
  71 }
  72
  73 /// Returns the obligations that make this trait reference
  74 /// well-formed.  For example, if there is a trait `Set` defined like
  75 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  76 /// if `Bar: Eq`.
  77 pub fn trait_obligations<'a, 'tcx>(
  78     infcx: &InferCtxt<'a, 'tcx>,
  79     param_env: ty::ParamEnv<'tcx>,
  80     body_id: hir::HirId,
  81     trait_ref: &ty::TraitRef<'tcx>,
  82     span: Span,
  83     item: Option<&'tcx hir::Item<'tcx>>,
  84 ) -> Vec<traits::PredicateObligation<'tcx>> {
  85     let mut wf =
  86         WfPredicates { infcx, param_env, body_id, span, out: vec![], recursion_depth: 0, item };
  87     wf.compute_trait_ref(trait_ref, Elaborate::All);
  88     wf.normalize()
  89 }
  90
  91 pub fn predicate_obligations<'a, 'tcx>(
  92     infcx: &InferCtxt<'a, 'tcx>,
  93     param_env: ty::ParamEnv<'tcx>,
  94     body_id: hir::HirId,
  95     predicate: ty::Predicate<'tcx>,
  96     span: Span,
  97 ) -> Vec<traits::PredicateObligation<'tcx>> {
  98     let mut wf = WfPredicates {
  99         infcx,
 100         param_env,
 101         body_id,
 102         span,
 103         out: vec![],
 104         recursion_depth: 0,
 105         item: None,
 106     };
 107
 108     // It's ok to skip the binder here because wf code is prepared for it
 109     match predicate.skip_binders() {
 110         ty::PredicateAtom::Trait(t, _) => {
 111             wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
 112         }
 113         ty::PredicateAtom::RegionOutlives(..) => {}
 114         ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 115             wf.compute(ty.into());
 116         }
 117         ty::PredicateAtom::Projection(t) => {
 118             wf.compute_projection(t.projection_ty);
 119             wf.compute(t.ty.into());
 120         }
 121         ty::PredicateAtom::WellFormed(arg) => {
 122             wf.compute(arg);
 123         }
 124         ty::PredicateAtom::ObjectSafe(_) => {}
 125         ty::PredicateAtom::ClosureKind(..) => {}
 126         ty::PredicateAtom::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 127             wf.compute(a.into());
 128             wf.compute(b.into());
 129         }
 130         ty::PredicateAtom::ConstEvaluatable(def, substs) => {
 131             let obligations = wf.nominal_obligations(def.did, substs);
 132             wf.out.extend(obligations);
 133
 134             for arg in substs.iter() {
 135                 wf.compute(arg);
 136             }
 137         }
 138         ty::PredicateAtom::ConstEquate(c1, c2) => {
 139             wf.compute(c1.into());
 140             wf.compute(c2.into());
 141         }
 142         ty::PredicateAtom::TypeWellFormedFromEnv(..) => {
 143             bug!("TypeWellFormedFromEnv is only used for Chalk")
 144         }
 145     }
 146
 147     wf.normalize()
 148 }
 149
 150 struct WfPredicates<'a, 'tcx> {
 151     infcx: &'a InferCtxt<'a, 'tcx>,
 152     param_env: ty::ParamEnv<'tcx>,
 153     body_id: hir::HirId,
 154     span: Span,
 155     out: Vec<traits::PredicateObligation<'tcx>>,
 156     recursion_depth: usize,
 157     item: Option<&'tcx hir::Item<'tcx>>,
 158 }
 159
 160 /// Controls whether we "elaborate" supertraits and so forth on the WF
 161 /// predicates. This is a kind of hack to address #43784. The
 162 /// underlying problem in that issue was a trait structure like:
 163 ///
 164 /// ```
 165 /// trait Foo: Copy { }
 166 /// trait Bar: Foo { }
 167 /// impl<T: Bar> Foo for T { }
 168 /// impl<T> Bar for T { }
 169 /// ```
 170 ///
 171 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 172 /// we decide that this is true because `T: Bar` is in the
 173 /// where-clauses (and we can elaborate that to include `T:
 174 /// Copy`). This wouldn't be a problem, except that when we check the
 175 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 176 /// impl. And so nowhere did we check that `T: Copy` holds!
 177 ///
 178 /// To resolve this, we elaborate the WF requirements that must be
 179 /// proven when checking impls. This means that (e.g.) the `impl Bar
 180 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 181 /// Copy` (which it won't be able to do, because there is no `Copy`
 182 /// impl for `T`).
 183 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 184 enum Elaborate {
 185     All,
 186     None,
 187 }
 188
 189 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 190     tcx: TyCtxt<'tcx>,
 191     trait_ref: &ty::TraitRef<'tcx>,
 192     item: Option<&hir::Item<'tcx>>,
 193     cause: &mut traits::ObligationCause<'tcx>,
 194     pred: &ty::Predicate<'tcx>,
 195     mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>,
 196 ) {
 197     debug!(
 198         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 199         trait_ref, item, cause, pred
 200     );
 201     let items = match item {
 202         Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
 203         _ => return,
 204     };
 205     let fix_span =
 206         |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
 207             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 208             _ => impl_item_ref.span,
 209         };
 210
 211     // It is fine to skip the binder as we don't care about regions here.
 212     match pred.skip_binders() {
 213         ty::PredicateAtom::Projection(proj) => {
 214             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 215             // but rather from a "second order" obligation, where an associated type has a
 216             // projection coming from another associated type. See
 217             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 218             // `traits-assoc-type-in-supertrait-bad.rs`.
 219             if let ty::Projection(projection_ty) = proj.ty.kind() {
 220                 let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
 221                 if let Some(impl_item_span) =
 222                     items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 223                 {
 224                     cause.make_mut().span = impl_item_span;
 225                 }
 226             }
 227         }
 228         ty::PredicateAtom::Trait(pred, _) => {
 229             // An associated item obligation born out of the `trait` failed to be met. An example
 230             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 231             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 232             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind() {
 233                 if let Some(impl_item_span) = trait_assoc_items
 234                     .find(|i| i.def_id == item_def_id)
 235                     .and_then(|trait_assoc_item| {
 236                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
 237                     })
 238                 {
 239                     cause.make_mut().span = impl_item_span;
 240                 }
 241             }
 242         }
 243         _ => {}
 244     }
 245 }
 246
 247 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 248     fn tcx(&self) -> TyCtxt<'tcx> {
 249         self.infcx.tcx
 250     }
 251
 252     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 253         traits::ObligationCause::new(self.span, self.body_id, code)
 254     }
 255
 256     fn normalize(mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 257         let cause = self.cause(traits::MiscObligation);
 258         let infcx = &mut self.infcx;
 259         let param_env = self.param_env;
 260         let mut obligations = Vec::with_capacity(self.out.len());
 261         for mut obligation in self.out {
 262             assert!(!obligation.has_escaping_bound_vars());
 263             let mut selcx = traits::SelectionContext::new(infcx);
 264             // Don't normalize the whole obligation, the param env is either
 265             // already normalized, or we're currently normalizing the
 266             // param_env. Either way we should only normalize the predicate.
 267             let normalized_predicate = traits::project::normalize_with_depth_to(
 268                 &mut selcx,
 269                 param_env,
 270                 cause.clone(),
 271                 self.recursion_depth,
 272                 &obligation.predicate,
 273                 &mut obligations,
 274             );
 275             obligation.predicate = normalized_predicate;
 276             obligations.push(obligation);
 277         }
 278         obligations
 279     }
 280
 281     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 282     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 283         let tcx = self.infcx.tcx;
 284         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 285
 286         debug!("compute_trait_ref obligations {:?}", obligations);
 287         let cause = self.cause(traits::MiscObligation);
 288         let param_env = self.param_env;
 289         let depth = self.recursion_depth;
 290
 291         let item = self.item;
 292
 293         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 294             let mut cause = cause.clone();
 295             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
 296                 let derived_cause = traits::DerivedObligationCause {
 297                     parent_trait_ref,
 298                     parent_code: Rc::new(obligation.cause.code.clone()),
 299                 };
 300                 cause.make_mut().code =
 301                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 302             }
 303             extend_cause_with_original_assoc_item_obligation(
 304                 tcx,
 305                 trait_ref,
 306                 item,
 307                 &mut cause,
 308                 &obligation.predicate,
 309                 tcx.associated_items(trait_ref.def_id).in_definition_order(),
 310             );
 311             traits::Obligation::with_depth(cause, depth, param_env, obligation.predicate)
 312         };
 313
 314         if let Elaborate::All = elaborate {
 315             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 316             let implied_obligations = implied_obligations.map(extend);
 317             self.out.extend(implied_obligations);
 318         } else {
 319             self.out.extend(obligations);
 320         }
 321
 322         let tcx = self.tcx();
 323         self.out.extend(
 324             trait_ref
 325                 .substs
 326                 .iter()
 327                 .enumerate()
 328                 .filter(|(_, arg)| {
 329                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 330                 })
 331                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 332                 .map(|(i, arg)| {
 333                     let mut new_cause = cause.clone();
 334                     // The first subst is the self ty - use the correct span for it.
 335                     if i == 0 {
 336                         if let Some(hir::ItemKind::Impl { self_ty, .. }) = item.map(|i| &i.kind) {
 337                             new_cause.make_mut().span = self_ty.span;
 338                         }
 339                     }
 340                     traits::Obligation::with_depth(
 341                         new_cause,
 342                         depth,
 343                         param_env,
 344                         ty::PredicateAtom::WellFormed(arg).to_predicate(tcx),
 345                     )
 346                 }),
 347         );
 348     }
 349
 350     /// Pushes the obligations required for `trait_ref::Item` to be WF
 351     /// into `self.out`.
 352     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 353         // A projection is well-formed if
 354         //
 355         // (a) its predicates hold (*)
 356         // (b) its substs are wf
 357         //
 358         // (*) The predicates of an associated type include the predicates of
 359         //     the trait that it's contained in. For example, given
 360         //
 361         // trait A<T>: Clone {
 362         //     type X where T: Copy;
 363         // }
 364         //
 365         // The predicates of `<() as A<i32>>::X` are:
 366         // [
 367         //     `(): Sized`
 368         //     `(): Clone`
 369         //     `(): A<i32>`
 370         //     `i32: Sized`
 371         //     `i32: Clone`
 372         //     `i32: Copy`
 373         // ]
 374         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 375         self.out.extend(obligations);
 376
 377         let tcx = self.tcx();
 378         let cause = self.cause(traits::MiscObligation);
 379         let param_env = self.param_env;
 380         let depth = self.recursion_depth;
 381
 382         self.out.extend(
 383             data.substs
 384                 .iter()
 385                 .filter(|arg| {
 386                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 387                 })
 388                 .filter(|arg| !arg.has_escaping_bound_vars())
 389                 .map(|arg| {
 390                     traits::Obligation::with_depth(
 391                         cause.clone(),
 392                         depth,
 393                         param_env,
 394                         ty::PredicateAtom::WellFormed(arg).to_predicate(tcx),
 395                     )
 396                 }),
 397         );
 398     }
 399
 400     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 401         if !subty.has_escaping_bound_vars() {
 402             let cause = self.cause(cause);
 403             let trait_ref = ty::TraitRef {
 404                 def_id: self.infcx.tcx.require_lang_item(LangItem::Sized, None),
 405                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 406             };
 407             self.out.push(traits::Obligation::with_depth(
 408                 cause,
 409                 self.recursion_depth,
 410                 self.param_env,
 411                 trait_ref.without_const().to_predicate(self.infcx.tcx),
 412             ));
 413         }
 414     }
 415
 416     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 417     fn compute(&mut self, arg: GenericArg<'tcx>) {
 418         let mut walker = arg.walk();
 419         let param_env = self.param_env;
 420         let depth = self.recursion_depth;
 421         while let Some(arg) = walker.next() {
 422             let ty = match arg.unpack() {
 423                 GenericArgKind::Type(ty) => ty,
 424
 425                 // No WF constraints for lifetimes being present, any outlives
 426                 // obligations are handled by the parent (e.g. `ty::Ref`).
 427                 GenericArgKind::Lifetime(_) => continue,
 428
 429                 GenericArgKind::Const(constant) => {
 430                     match constant.val {
 431                         ty::ConstKind::Unevaluated(def, substs, promoted) => {
 432                             assert!(promoted.is_none());
 433
 434                             let obligations = self.nominal_obligations(def.did, substs);
 435                             self.out.extend(obligations);
 436
 437                             let predicate = ty::PredicateAtom::ConstEvaluatable(def, substs)
 438                                 .to_predicate(self.tcx());
 439                             let cause = self.cause(traits::MiscObligation);
 440                             self.out.push(traits::Obligation::with_depth(
 441                                 cause,
 442                                 self.recursion_depth,
 443                                 self.param_env,
 444                                 predicate,
 445                             ));
 446                         }
 447                         ty::ConstKind::Infer(infer) => {
 448                             let resolved = self.infcx.shallow_resolve(infer);
 449                             // the `InferConst` changed, meaning that we made progress.
 450                             if resolved != infer {
 451                                 let cause = self.cause(traits::MiscObligation);
 452
 453                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 454                                     val: ty::ConstKind::Infer(resolved),
 455                                     ..*constant
 456                                 });
 457                                 self.out.push(traits::Obligation::with_depth(
 458                                     cause,
 459                                     self.recursion_depth,
 460                                     self.param_env,
 461                                     ty::PredicateAtom::WellFormed(resolved_constant.into())
 462                                         .to_predicate(self.tcx()),
 463                                 ));
 464                             }
 465                         }
 466                         ty::ConstKind::Error(_)
 467                         | ty::ConstKind::Param(_)
 468                         | ty::ConstKind::Bound(..)
 469                         | ty::ConstKind::Placeholder(..) => {
 470                             // These variants are trivially WF, so nothing to do here.
 471                         }
 472                         ty::ConstKind::Value(..) => {
 473                             // FIXME: Enforce that values are structurally-matchable.
 474                         }
 475                     }
 476                     continue;
 477                 }
 478             };
 479
 480             match *ty.kind() {
 481                 ty::Bool
 482                 | ty::Char
 483                 | ty::Int(..)
 484                 | ty::Uint(..)
 485                 | ty::Float(..)
 486                 | ty::Error(_)
 487                 | ty::Str
 488                 | ty::GeneratorWitness(..)
 489                 | ty::Never
 490                 | ty::Param(_)
 491                 | ty::Bound(..)
 492                 | ty::Placeholder(..)
 493                 | ty::Foreign(..) => {
 494                     // WfScalar, WfParameter, etc
 495                 }
 496
 497                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 498                 ty::Infer(ty::IntVar(_)) => {}
 499
 500                 // Can only infer to `ty::Float(_)`.
 501                 ty::Infer(ty::FloatVar(_)) => {}
 502
 503                 ty::Slice(subty) => {
 504                     self.require_sized(subty, traits::SliceOrArrayElem);
 505                 }
 506
 507                 ty::Array(subty, _) => {
 508                     self.require_sized(subty, traits::SliceOrArrayElem);
 509                     // Note that we handle the len is implicitly checked while walking `arg`.
 510                 }
 511
 512                 ty::Tuple(ref tys) => {
 513                     if let Some((_last, rest)) = tys.split_last() {
 514                         for elem in rest {
 515                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 516                         }
 517                     }
 518                 }
 519
 520                 ty::RawPtr(_) => {
 521                     // Simple cases that are WF if their type args are WF.
 522                 }
 523
 524                 ty::Projection(data) => {
 525                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 526                     self.compute_projection(data);
 527                 }
 528
 529                 ty::Adt(def, substs) => {
 530                     // WfNominalType
 531                     let obligations = self.nominal_obligations(def.did, substs);
 532                     self.out.extend(obligations);
 533                 }
 534
 535                 ty::FnDef(did, substs) => {
 536                     let obligations = self.nominal_obligations(did, substs);
 537                     self.out.extend(obligations);
 538                 }
 539
 540                 ty::Ref(r, rty, _) => {
 541                     // WfReference
 542                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 543                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 544                         self.out.push(traits::Obligation::with_depth(
 545                             cause,
 546                             depth,
 547                             param_env,
 548                             ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(rty, r))
 549                                 .to_predicate(self.tcx()),
 550                         ));
 551                     }
 552                 }
 553
 554                 ty::Generator(..) => {
 555                     // Walk ALL the types in the generator: this will
 556                     // include the upvar types as well as the yield
 557                     // type. Note that this is mildly distinct from
 558                     // the closure case, where we have to be careful
 559                     // about the signature of the closure. We don't
 560                     // have the problem of implied bounds here since
 561                     // generators don't take arguments.
 562                 }
 563
 564                 ty::Closure(_, substs) => {
 565                     // Only check the upvar types for WF, not the rest
 566                     // of the types within. This is needed because we
 567                     // capture the signature and it may not be WF
 568                     // without the implied bounds. Consider a closure
 569                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 570                     // not known to hold in the creator's context (and
 571                     // indeed the closure may not be invoked by its
 572                     // creator, but rather turned to someone who *can*
 573                     // verify that).
 574                     //
 575                     // The special treatment of closures here really
 576                     // ought not to be necessary either; the problem
 577                     // is related to #25860 -- there is no way for us
 578                     // to express a fn type complete with the implied
 579                     // bounds that it is assuming. I think in reality
 580                     // the WF rules around fn are a bit messed up, and
 581                     // that is the rot problem: `fn(&'a T)` should
 582                     // probably always be WF, because it should be
 583                     // shorthand for something like `where(T: 'a) {
 584                     // fn(&'a T) }`, as discussed in #25860.
 585                     //
 586                     // Note that we are also skipping the generic
 587                     // types. This is consistent with the `outlives`
 588                     // code, but anyway doesn't matter: within the fn
 589                     // body where they are created, the generics will
 590                     // always be WF, and outside of that fn body we
 591                     // are not directly inspecting closure types
 592                     // anyway, except via auto trait matching (which
 593                     // only inspects the upvar types).
 594                     walker.skip_current_subtree(); // subtree handled below
 595                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 596                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 597                 }
 598
 599                 ty::FnPtr(_) => {
 600                     // let the loop iterate into the argument/return
 601                     // types appearing in the fn signature
 602                 }
 603
 604                 ty::Opaque(did, substs) => {
 605                     // all of the requirements on type parameters
 606                     // should've been checked by the instantiation
 607                     // of whatever returned this exact `impl Trait`.
 608
 609                     // for named opaque `impl Trait` types we still need to check them
 610                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 611                         let obligations = self.nominal_obligations(did, substs);
 612                         self.out.extend(obligations);
 613                     }
 614                 }
 615
 616                 ty::Dynamic(data, r) => {
 617                     // WfObject
 618                     //
 619                     // Here, we defer WF checking due to higher-ranked
 620                     // regions. This is perhaps not ideal.
 621                     self.from_object_ty(ty, data, r);
 622
 623                     // FIXME(#27579) RFC also considers adding trait
 624                     // obligations that don't refer to Self and
 625                     // checking those
 626
 627                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 628
 629                     if !defer_to_coercion {
 630                         let cause = self.cause(traits::MiscObligation);
 631                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 632                         let tcx = self.tcx();
 633                         self.out.extend(component_traits.map(|did| {
 634                             traits::Obligation::with_depth(
 635                                 cause.clone(),
 636                                 depth,
 637                                 param_env,
 638                                 ty::PredicateAtom::ObjectSafe(did).to_predicate(tcx),
 639                             )
 640                         }));
 641                     }
 642                 }
 643
 644                 // Inference variables are the complicated case, since we don't
 645                 // know what type they are. We do two things:
 646                 //
 647                 // 1. Check if they have been resolved, and if so proceed with
 648                 //    THAT type.
 649                 // 2. If not, we've at least simplified things (e.g., we went
 650                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 651                 //    register a pending obligation and keep
 652                 //    moving. (Goal is that an "inductive hypothesis"
 653                 //    is satisfied to ensure termination.)
 654                 // See also the comment on `fn obligations`, describing "livelock"
 655                 // prevention, which happens before this can be reached.
 656                 ty::Infer(_) => {
 657                     let ty = self.infcx.shallow_resolve(ty);
 658                     if let ty::Infer(ty::TyVar(_)) = ty.kind() {
 659                         // Not yet resolved, but we've made progress.
 660                         let cause = self.cause(traits::MiscObligation);
 661                         self.out.push(traits::Obligation::with_depth(
 662                             cause,
 663                             self.recursion_depth,
 664                             param_env,
 665                             ty::PredicateAtom::WellFormed(ty.into()).to_predicate(self.tcx()),
 666                         ));
 667                     } else {
 668                         // Yes, resolved, proceed with the result.
 669                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 670                         self.compute(ty.into());
 671                     }
 672                 }
 673             }
 674         }
 675     }
 676
 677     fn nominal_obligations(
 678         &mut self,
 679         def_id: DefId,
 680         substs: SubstsRef<'tcx>,
 681     ) -> Vec<traits::PredicateObligation<'tcx>> {
 682         let predicates = self.infcx.tcx.predicates_of(def_id);
 683         let mut origins = vec![def_id; predicates.predicates.len()];
 684         let mut head = predicates;
 685         while let Some(parent) = head.parent {
 686             head = self.infcx.tcx.predicates_of(parent);
 687             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 688         }
 689
 690         let predicates = predicates.instantiate(self.infcx.tcx, substs);
 691         debug_assert_eq!(predicates.predicates.len(), origins.len());
 692
 693         predicates
 694             .predicates
 695             .into_iter()
 696             .zip(predicates.spans.into_iter())
 697             .zip(origins.into_iter().rev())
 698             .map(|((pred, span), origin_def_id)| {
 699                 let cause = self.cause(traits::BindingObligation(origin_def_id, span));
 700                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 701             })
 702             .filter(|pred| !pred.has_escaping_bound_vars())
 703             .collect()
 704     }
 705
 706     fn from_object_ty(
 707         &mut self,
 708         ty: Ty<'tcx>,
 709         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 710         region: ty::Region<'tcx>,
 711     ) {
 712         // Imagine a type like this:
 713         //
 714         //     trait Foo { }
 715         //     trait Bar<'c> : 'c { }
 716         //
 717         //     &'b (Foo+'c+Bar<'d>)
 718         //         ^
 719         //
 720         // In this case, the following relationships must hold:
 721         //
 722         //     'b <= 'c
 723         //     'd <= 'c
 724         //
 725         // The first conditions is due to the normal region pointer
 726         // rules, which say that a reference cannot outlive its
 727         // referent.
 728         //
 729         // The final condition may be a bit surprising. In particular,
 730         // you may expect that it would have been `'c <= 'd`, since
 731         // usually lifetimes of outer things are conservative
 732         // approximations for inner things. However, it works somewhat
 733         // differently with trait objects: here the idea is that if the
 734         // user specifies a region bound (`'c`, in this case) it is the
 735         // "master bound" that *implies* that bounds from other traits are
 736         // all met. (Remember that *all bounds* in a type like
 737         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 738         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 739         // 'y.)
 740         //
 741         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 742         // am looking forward to the future here.
 743         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 744             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 745
 746             let explicit_bound = region;
 747
 748             self.out.reserve(implicit_bounds.len());
 749             for implicit_bound in implicit_bounds {
 750                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 751                 let outlives =
 752                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 753                 self.out.push(traits::Obligation::with_depth(
 754                     cause,
 755                     self.recursion_depth,
 756                     self.param_env,
 757                     outlives.to_predicate(self.infcx.tcx),
 758                 ));
 759             }
 760         }
 761     }
 762 }
 763
 764 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 765 /// bounds that must hold on the elided self type. These are derived
 766 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 767 /// they declare `trait SomeTrait : 'static`, for example, then
 768 /// `'static` would appear in the list. The hard work is done by
 769 /// `infer::required_region_bounds`, see that for more information.
 770 pub fn object_region_bounds<'tcx>(
 771     tcx: TyCtxt<'tcx>,
 772     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 773 ) -> Vec<ty::Region<'tcx>> {
 774     // Since we don't actually *know* the self type for an object,
 775     // this "open(err)" serves as a kind of dummy standin -- basically
 776     // a placeholder type.
 777     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 778
 779     let predicates = existential_predicates.iter().filter_map(|predicate| {
 780         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 781             None
 782         } else {
 783             Some(predicate.with_self_ty(tcx, open_ty))
 784         }
 785     });
 786
 787     required_region_bounds(tcx, open_ty, predicates)
 788 }